The present invention relates to a pneumatically operable screw driver preferably used for inserting a threaded fastening member into a board member such as a wood material or the like.
FIG. 6 shows a conventional screw driver disclosed in the unexamined Japanese utility model publication No. 61-75966. This conventional screw driver has an air motor 102 and a driver bit 109 driven by a driving force of the air motor 102. The rotation of the air motor 102 is transmitted via a speed-reduction mechanism 120 to an anvil 110. The speed-reduction mechanism 120 is constituted by a planetary gear 172 or the like. The driver bit 109 is detachably engaged with the front end of the anvil 110. The driver bit 109 and the anvil 110 are slidable in the axial direction of a cylindrical body 101 of the screw driver. The driver bit 109 has a tip engageable with a head 131 of a screw 130. The operator pushes the screw driver body 101 in the axial direction. A pressing force applied on the driver bit 109 acts against the screw 130 placed at a position corresponding to an engaging hole 129a of a board member 129.
In this case, the driver bit 109 receives a reaction force from the screw 130 pressed against the board member 129. The driver bit 109 thus causes a retractile (i.e., rearward) shift movement relative to the screw driver body 101. The driver bit 109 and the anvil 110 shift together in the axial direction of the screw driver body 101. An operation rod 117 has a front end inserted in an engagement bore formed at the rear end of the anvil 110. In response to the retractile shift movement of the anvil 110, the operation rod 117 lifts an intake valve 107 upward. An intake port 104 is provided at the rearmost end of the screw driver. When the intake valve 107 is lifted upward, an air passage 106a connects the intake port 104 to the air motor 102 so as to supply the compression air into the air motor 102. The air motor 102 starts its operation.
In this manner, the air motor 102 is activated in response to the retractile shift movement of the driver bit 109 (and the anvil 110) relative to the screw driver body 101. When the screw driving operation is finished, the operator releases the pushing force applied on the screw driver body 101. Thus, the driver bit 109 shifts oppositely in the axial direction relative to the screw driver body 101 and returns to the original position. The operation rod 117 also returns to its original position. Thus, the intake valve 107 moves downward to close the air passage 106a. No compression air is supplied to the air motor 102. The air motor 102 is stopped.
A driver guide 112 has a cylindrical body with a rear end threaded and engageable with a cylindrical inner wall of a front sleeve of the screw driver body 101. The driver guide 112 has an axial hole along which the driver bit 109 is slidable in the back-and-forth direction. The axial position of the driver guide 112 with respect to the screw driver body 101 is changeable by rotating the driver guide 112 about its axis. In other words, the length of the driver bit 109 protruding from the front end of the driver guide 112 is adjustable by rotating the driver guide 112. Accordingly, the driver guide 112 makes it possible to restrict the fastening depth of the screw 130 to a constant value.
The screw driving operation of the above-described conventional screw driver will be explained with reference to FIGS. 7A to 7C. In this case, the axial position of the driver guide 112 is adjusted beforehand to optimize the protrusile length of the driver bit 109 to a designated position. Through this adjustment using the driver guide 112, when the screw 130 is completely inserted into the board member 129 by the driver bit 109, the head 131 of the screw 130 becomes flush with the upper surface of the board member 129.
First, in the beginning of the screw driving (or fastening) operation, a cross-shaped (ridged) tip of the driver bit 109 is engaged with a corresponding cross groove formed on the head 131 of the screw 130. The operator pushes the screw driver body 101 in the axial direction to press the driver bit 109 against the screw 130 placed in the engaging hole 129a of the board member 129. The operation rod 117 receives the reaction force from the board member 129 via the screw 130, the driver bit 109 and the anvil 110. The operation rod 117 is thus shifted upward to open the intake valve 107. Upon opening the intake valve 107, the compression air flows into the air motor 102 from the intake port 104 via the air passage 106a. The air motor 102 starts rotating. The driver bit 109 rotates to fasten the screw 130 into board member 129, as shown in FIG. 7A.
During the screw driving operation, the front end of the driver guide 112 comes to contact with the board member 129 when the screw head 131 reaches an altitudinal height "d" from the board member 129, as shown in FIG. 7B. The distance "d" is identical with an opening clearance of the intake valve 107. The opening clearance of the intake valve 107 is defined by the axial lift amount of the intake valve 107. After the driver guide 112 is brought into contact with the board member 129, the driver bit 109 does not receive the reaction force from the board member 129. At this moment, the screw 130 is still driven into the board member 129. The driver bit 109 continues driving the screw 130 forward until the intake valve 107 is closed. After the driver bit 109 advances forward together with the operation rod 117 by an amount equivalent to the clearance "d", the intake valve 107 is closed as shown in FIG. 7C. The air motor 102 is stopped. At this moment, the screw head 131 is positioned in flush with the upper surface of the board member 129. The screw driving operation is completed in this manner.
The above-described screw driver is generally referred to as "push-start type screw driver" characterized in that the air motor 102 is automatically activated by pushing the screw driver body 101 under the condition where the driver bit 109 is engaged with the screw 130. This realizes the speedy handling of the screw driver, improving the workability. The provision of the driver guide 112 makes it possible to restrict the fastening depth of the screw 130 to a constant value, assuring the good finish in the screw driving operation.
However, the generally used screw is a Phillips type screw having on its head a recess in the shape of a cross. The operator needs to continuously apply a predetermined torque on the driver bit 109 engaged with the cross groove on the screw head 131. If the torque applied on the driver bit 109 is smaller than this predetermined torque, the driver bit 109 will shift upward due to the reaction force caused by the fastening torque of the driver bit 109 itself. Thus, the driver bit 109 tends to exit out of the cross groove of the screw head 131. This behavior is generally referred to as a "come-out" phenomenon which causes the slipping engagement between the driver bit 109 and the screw head 131. The "come-out" phenomenon may damage the cross groove on the screw head 131. The tip of the driver bit 109 will wear at an early stage.
In general, it is possible to suppress the "come-out" phenomenon as long as the driver bit 109 and the anvil 110 are positioned at the uppermost position with a sufficient pressing force applied on the screw driver.
As described above, using the driver guide 112 is effective to obtain a constant fastening depth. However, the presence of the driver guide 112 possibly causes the "come-out" phenomenon. As explained with reference to FIG. 7B, the driver bit 109 does not receive a sufficient reaction force from the screw 130 after the driver guide 112 is brought into contact with the board member 129. During the remaining fastening operation from the condition of FIG. 7B to the condition of FIG. 7C, the driver bit 109 causes a protrusile shift movement together with the anvil 110 relative to the screw driver body 101. In this case, the driver bit 109 continues fastening the screw 130 with a pressing force applied on the anvil 110 by the spring 118 provided above the intake valve 107. The resilient force of the spring 118 is relatively small. Accordingly, in the final fastening operation (i.e., the protrusile shift movement of the driver bit 109 and the anvil 110) from the condition of FIG. 7B to the condition of FIG. 7C, the driver bit 109 and the anvil 110 may cause an undesirable retractile shift movement relative to the screw driver body 101 due to the reaction force caused by the fastening torque of the driver bit 109 itself. Thus, the "come-out" phenomenon is possibly caused in the final stage of the screw driving operation.
Furthermore, the board member 129 may be made of a soft material, such as a gypsum or plaster board. In such cases, the soft board member 129 may induce the "come-out" phenomenon. The screw 130 is easily inserted into the soft board member 129. The driver bit 109 will not receive a sufficient reaction force from the screw 130 if the fastening speed of the driver bit 109 is slow.
The spring 118, provided above the intake valve 107, always urges the driver bit 109 and the anvil 110 downward. To prevent the "come-out" phenomenon, it is possible to set the load of the spring 118 to a larger value exceeding the reaction force of the fastening torque. However, such a setting forces the operator to strongly push the screw driver against an excessively large force equivalent to the increased resilient force of the spring 118. The operability of the screw driver is significantly worsened.